1. Field of the Invention
The present invention relates to a semiconductor device that uses, as an active layer, a semiconductor thin film formed on a base member having an insulating surface and, more specifically, to a thin-film transistor that uses a crystalline silicon film as an active layer.
2. Description of the Related Art
In recent years, the technology for forming a thin-film transistor (TFT) by using a semiconductor thin film (thickness: hundreds to thousands of angstrom) that is formed on a base member having an insulating substrate attract much attention. The thin-film transistor is widely applied to electronic devices such as ICs and electro-optical devices and, in particular, is now being developed rapidly as an switching element of image display devices.
For example, in a liquid crystal display device, it is attempted to apply TFTs to every electric circuit such as a pixel matrix circuit for controlling individual pixel regions that are arranged in matrix form, a driver circuit for controlling the pixel matrix circuit, or a logic circuit (a processor circuit, a memory circuit, or the like) for processing external data signals.
At present, TFTs have been put into practical use that use an amorphous silicon film as an active layer. However, TFTs that use a crystalline silicon film (polysilicon film) are needed for electric circuits that are required to operate even faster, such as a driver circuit and a logic circuit.
The technique disclosed in Japanese Unexamined Patent Publication Nos. Hei. 6-232059 and Hei. 6-244103 is known as a method for forming a crystalline silicon film on a base member. These technique enables formation of a crystalline silicon film that is superior in crystallinity through a heat treatment of 500xc2x0-600xc2x0 C. and about 4 hours by utilizing a metal element (particularly nickel) for accelerating crystallization of silicon.
However, even if a driver circuit is constructed by using such TFTs, it does not completely satisfy the required performance. In particular, it, is still impossible to construct, by conventional TFTs, high-speed logic circuits in which extremely high electrical performance is required to realize high-speed operation and a high breakdown voltage characteristic at the same time.
Accordingly, to improve the performance of electro-optical devices etc., it is necessary to realize a TFT whose performance is equivalent to that of a MOSFET formed by using a single crystal silicon wafer.
An object of the invention is therefore to provide a thin-film semiconductor device having extremely high performance and a manufacturing method thereof as an breakthrough for enabling further improvement of the performance of electro-optical devices.
As for the reason why the conventional method cannot provide a high-performance TFT as mentioned above, it is considered carriers (electrons or holes) are captured at grain boundaries and, as a result, the field-effect mobility that is one of the parameters indicating the TFT characteristics is prevented from being increased.
For example, dangling bonds of silicon atoms and defect (trap) states exist in a large number at grain boundaries. Carriers traveling through the inside of each crystal are easily trapped by dangling bonds, defect states, or the like when they approach or contact the grain boundaries. Therefore, it is considered that the grain boundaries behave as xe2x80x9cmalignant grain boundariesxe2x80x9d that obstruct carrier movement.
To realize a high-performance semiconductor device as mentioned above, a technique is indispensable that changes the structure of xe2x80x9cmalignant grain boundariesxe2x80x9d to convert them into xe2x80x9cbenign grain boundariesxe2x80x9d for carriers. That is, it can be said that it is important to form grain boundaries at least having a low possibility of capturing carriers, that is, a low possibility of obstructing carrier movement.
Accordingly, the invention provides a manufacturing method of a semiconductor device having an active layer that is a semiconductor thin film, comprising the steps of forming an amorphous silicon film on a base member having an insulating surface; holding a metal element for accelerating crystallization in a given positional relationship with the amorphous silicon film; converting the amorphous silicon film into a crystalline silicon film by a first heat treatment; patterning the crystalline silicon film into an active layer; forming a gate insulating film on the active layer; performing a second heat treatment in an atmosphere containing a halogen element, to thereby remove the metal element from the active layer by gettering and to form a thermal oxidation film at an interface between the active layer and the gate insulating film; and performing a third heat treatment in a nitrogen atmosphere, to thereby improve a film quality and an interface state of the gate insulating film including the thermal oxidation film, wherein grain boundaries in the active layer have directivity and the active layer is a collection of a plurality of needle-like or columnar crystals extending generally parallel with the base member.
If a crystalline silicon film is formed according to the above manufacturing method, a thin film is formed that has an appearance as shown in FIG. 13, which is a microscope photograph of a crystalline silicon film as enlarged. As seen from FIG. 13, the crystalline silicon film is a collection of a plurality of crystal grains having as large diameters as tens of micrometers to a little larger than 100 xcexcm. This manufacturing method utilizes, as a means for crystallizing an amorphous silicon film, the technique disclosed in Japanese Unexamined Patent Publication No. Hei. 6-232059.
FIG. 14 is a TEM photograph of a minute region as enlarged of the inside of a crystal grain, which was taken to scrutinize the inside of individual crystal grains shown in FIG. 13.
That is, the crystalline silicon film according to the invention macroscopically appears like a collection of large grains as shown in FIG. 13, actually its inside is a crystal structural body as a collection of a plurality of needle-like or columnar crystals 1401 as shown in FIG. 14.
In FIG. 14, reference numeral 1402 denotes grain boundaries, i.e., boundaries between the needle-like or columnar crystals 1401. It is seen from the extending direction of the grain boundaries 1402 that the needle-like or columnar crystals 1401 grew generally parallel with each other. In this specification, the term xe2x80x9cgrain boundariesxe2x80x9d means boundaries between needle-like or columnar crystals unless otherwise specified.
In the active layer of the semiconductor device according to the invention, the metal element (principal example: nickel) for accelerating crystallization is gettering-removed by the heat treatment in the atmosphere containing a halogen element, so that the metal element that previously remained at a concentration higher than 1xc3x971018 atoms/cm3 is reduced to lower than or equal to 1xc3x971018 atoms/cm3, typically 1xc3x971014 to 5xc3x971017 atoms/cm3 (preferably lower than the spin density). Moreover, the phosphorous gettering method may be used for reducing the concentration of metal element in the semiconductor layer. The technique disclosed in U.S. patent publication Ser. No. 08/623,336 and Japanese Unexamined Patent Publication Hei. 8-340127 by Yamazaki et al. is known as a method for removing the metal element from the crystalline silicon.
It is naturally considered that other metal elements such as Cu, Al, etc. that were introduced by contamination or the like (that is, not introduced intentionally) are also removed by gettering.
At this time, it is expected that dangling bonds of silicon atoms connect to oxygen atoms during the heat treatment, to form the oxide (silicon oxide). It is considered that, as a result, silicon oxide is formed in the regions that were previously xe2x80x9cmalignant grain boundariesxe2x80x9d and substantially functions as grain boundaries.
It is presumed that the thus-formed grain boundaries 1402 are such that interfaces between silicon oxide and crystal silicon include almost no lattice defects, and hence are superior in matching performance. This is because interstitial silicon atoms that would otherwise cause defects are consumed by synergism of a process that silicon oxide is formed by thermal oxidation and a process that recombination between silicon atoms themselves or a silicon atom and an oxygen atom is accelerated by the catalytic effect of nickel.
That is, it is considered that the grain boundaries 1402 in FIG. 14 has almost no carrier-capturing defects and behave as xe2x80x9cbenign grain boundariesxe2x80x9d that function merely as energy barriers for carriers that move through the inside of the needle-like or columnar crystals 1401.
Since thermal oxidation reaction proceeds with preference in such grain boundaries, a thermal oxidation film that is formed there becomes thicker than in the other regions. Therefore, a gate voltage applied to the vicinities of the grain boundaries is apparently reduced, which also functions as energy barriers.
Since the heat treatment is performed at a relatively high temperature that is higher than 700xc2x0 C. (typically 800xc2x0-1,100xc2x0 C.), such crystal defects as dislocations and stacking faults that existed inside the needle-like or columnar crystals mostly disappear. Further, residual dangling bonds of silicon atoms are terminated by hydrogen atoms and halogen atoms contained in the film.
Thus, in the state of FIG. 14 that is obtained in the above manner, the inventors define the regions inside the plurality of needle-like or columnar crystals 1401 as xe2x80x9cregions that can substantially be regarded as a single crystal for carriers.xe2x80x9d
xe2x80x9cBeing substantially regarded as a single crystal for carriersxe2x80x9d means that there are no barriers that obstruct carrier movement, and can also be expressed as xe2x80x9cthere are no crystal defects or grain boundariesxe2x80x9d or xe2x80x9cthere are no potential barriers serving as energy barriers.xe2x80x9d
The invention is intended to form the active layer of a semiconductor device as typified by a TFT by utilizing the crystalline silicon film having the above structure, thereby realizing a high-performance semiconductor device suitable to construct a driver circuit and a logic circuit.
The invention as summarized above will be hereinafter described in detail in the form of various embodiments.